A typical trimmable power supply allows its output voltage to be adjusted slightly (or trimmed) up or down. Generally, an adjustment is made to the output voltage by changing a voltage at a dedicated output voltage adjustment pin. The voltage may be changed by changing an external resistance that is provided between the dedicated output voltage adjustment pin and a ground pin of the power supply or by applying an external voltage to the pin. For example, suppose a particular trimmable power supply provides 3.0 volts when 1K ohms is provided between its dedicated output voltage adjustment pin and ground pin. The output voltage may be trimmed to 2.7 volts by changing the resistance between these pins from 1K ohms to 500 ohms. Similarly, the output voltage may be trimmed to 3.3 volts by changing the resistance to 1.5K ohms.
Some trimmable power supplies generate output voltages using pulse width modulation (PWM). In such a power supply, an independent voltage source provides a power supply threshold voltage as a reference. Then, a PWM comparator provides a modulating pulse signal based on a comparison between the power supply threshold voltage and the power supply output voltage. The power supply output voltage is provided across a capacitor, and the modulating pulse signal opens and closes a switch that controls the charging of the capacitor. In particular, when the modulating pulse signal closes the switch, the capacitor charges and the power supply output voltage increases. On the other hand, when the modulating pulse signal opens the switch, no charge is provided to the capacitor and any externally connected circuits that draw charge from the capacitor cause the power supply output voltage to decrease.
Since externally connected circuits may draw charge from the capacitor, and since the capacitor charges only when the switch is closed, increasing the percentage of time in which the switch is closed (i.e., increasing the duty ratio of the modulating pulse signal) increases the power supply output voltage. Conversely, decreasing the percentage of time in which the switch is closed decreases the output voltage.
A typical trimmable power supply further includes a protection circuit that limits its output voltage. If the power supply output voltage exceeds the intended value by more than a certain percentage (e.g., 120%), the protection circuit typically deactivates (or shuts down) the power supply before it damages itself and any connected circuits. For example, a power supply that is adjusted to provide an output voltage of 3.0 volts may have an output voltage limit (or protection limit) of 3.6 volts. If the power supply operates improperly and attempts to provide more than 3.6 volts, the protection circuit of the power supply shuts down the power supply.
A typical power supply protection circuit includes a protection comparator having one input that receives the power supply output voltage, and another input that receives a protection threshold voltage (e.g., a voltage that is 120% of the intended output voltage). When the output voltage of the power supply exceeds the protection threshold voltage, the protection comparator generates a shutdown signal that shuts down the power supply (e.g., by preventing the PWM comparator from closing the switch that charges the capacitor providing the power supply output voltage). Generally, in a trimmable power supply that uses PWM, the same independent voltage source that provides the power supply threshold voltage to the PWM comparator also provides the protection threshold voltage to the protection comparator.
When trimming of the power supply output voltage occurs, trimming of the protection limit should be performed as well to preserve the same protection margin (e.g., 20%). For example, when the power supply output voltage is trimmed from 3.0 volts to 3.3 volts, the protection limit should be trimmed from 3.6 volts to 3.96 volts.
Since the protection limit is higher than the power supply output voltage, trimming adjustments generally are performed using separate adjustment pins. For example, when trimming the power supply voltage from 3.0 volts to 3.3 volts by changing a resistance from 1K ohms to 500 ohms between a dedicated output voltage adjustment pin and a ground pin, a corresponding trimming of the protection voltage from 3.6 volts to 3.96 volts typically occurs by changing a resistance from 1K ohms to 500 ohms between a dedicated protection limit adjustment pin and a ground pin.
Alternatively, the protection limit adjustment is made by changing a particular external voltage (rather than resistance) that is provided between a dedicated protection limit adjustment pin and a ground pin of the power supply.